Typical examples of liquid crystal display cells include a field effect mode cell proposed by M. Schadt et al. (APPLIED PHYSICS LETTERS, 18, 127-128 (1971)), a dynamic scattering mode cell proposed by G. H. Heilmeier et al. (PROCEEDING OF THE I.E.E.E., 56, 1162 -1171 (1968)) and a guest/host mode cell proposed by G. H. Heilmeier et al. (APPLIED PHYSICS LETTERS, 13, 91 (1968)) or D. L. White et al. (JOURNAL OF APPLIED PHYSICS, 45, 4718 (1974)).
Among these liquid crystal display cells, TN mode cells, which belong to the field effect mode cells, are majorly used at present. In the case of the TN mode cells, it is required to set the product of the optical anisotropy (.DELTA.n) of the liquid crystal material in the cell and the thickness (d; .mu.m) of the cell to a definite value in order to achieve good cell appearance, as indicated by G. Bauer (Mol. Cryst. Iiq. Cryst., 63, 45 (1981)). A liquid crystal display cell used in practice has a .DELTA.n.multidot.d value of either 0.5, 1.0, 1.6 or 2.2. Generally speaking, the visual properties of a liquid crystal display cell can be improved by setting the .DELTA.n.multidot.d value to 0.5. On the other hand, the frontal contrast thereof can be improved by setting the .DELTA.n.multidot.d value to 1.0, 1.6 or 2.2. Therefore it is generally recommended to set the .DELTA.n.multidot.d value of a liquid crystal display cell to 0.5, when it is regarded as important to achieve excellent visual properties from any direction. On the other hand, the .DELTA.n.multidot.d value thereof may be preferably set to 1.0, 1.6 or 2.2 in order to obtain a clear frontal contrast.
On the other hand, the thickness of a liquid crystal layer in a practically used liquid crystal display cell is commonly set to a definite value within a limited range of 6 to 10 .mu.m. Thus a liquid crysal material having a low .DELTA.n value is required in order to set the .DELTA.n.multidot.d value to 0.5. In contrast thereto, a liquid crystal material having a high .DELTA.n value is required in order to set the .DELTA.n.multidot.d value to 1.0, 1.6 or 2.2. Namely, either a liquid crystal material having a low .DELTA.n value or one having a high .DELTA.n value is required depending on the desired display properties.
On the other hand, most of practically available liquid crystal materials are prepared by mixing several or more components selected from among compounds showing a nematic phase at around room temperature and those showing a nematic phase at a temperature range higher than room temperature. Most of these mixed liquid crystals practically employed today are required to show a nematic phase over the whole temperature range of at least -30.degree. C. to +65.degree. C. Recent diversification of the application of liquid crystal display cells has brought about a demand for a liquid crystal material having a nematic liquid crystal temperature range enlarged toward the higher temperature. Thus it has been required to develop a nematic liquid crystal compound having a high nematic-to-isotropic (N-I) transition temperature.
However a nematic liquid crystal compound having a high N-I transition temperature frequently has a .DELTA.n value exceeding 0.12 (extrapolated value). Namely, the .DELTA.n value is too large to set the .DELTA.n.multidot.d value to 0.5. Accordingly it is required to deveolp a liquid crystal compound which has a high N-I transition temperature and yet a low .DELTA.n value in order to set the .DELTA.n.multidot.d value to 0.5.